Knowledge of the morphology, physiological properties, and supranuclear, internuclear, and intranuclear afferent and efferent synaptic connections of oculomotor motoneurones and internuclear neurones is an important prerequisite not only to understanding the neural mechanisms involved in normal oculomotor function, but also to interpreting the basis of functional deficits associated with strabismus and other oculomotor disorders. The morphology and synaptic relationships of axon collateral synaptic endings derived from motoneurones in the oculomotor nucleus, identified electrophysiologically and stained by intracellular injection of horseradish peroxidase, will be examined by light and electron microscopy. Oculomotor internuclear neurones will be distinguished on the basis of their efferent projections, distribution, ultrastructural features, synaptic organization, and neurotransmitter histochemical profiles. The soma-dendritic distribution of vestibular and abducens internuclear afferent synaptic endings on medial rectus motoneurones will be studied by light and electron microscope autoradiography. These findings will be compared to similar studies of abducens neurones as part of the long-term goal of elucidating the relationship between the neuronal and synaptic organization of the oculomotor nuclei and the control of eye movement.